This invention relates to fluid flow control through an electromagnetically actuated valve in a vehicle power steering system for speed dependent adjustment of standby type of auxiliary power steering arrangement.
The provision of a flow control system for use in power steering is already known as disclosed in German patent publication OS No. 29 48 228. In such prior art system, a step motor is energized under control of speed of travel to effect adjustment of a flow restrictor. A speed sensor delivers a digital signal for such purpose which is initially converted into analog form and by means of a Schmitt trigger utilized with different switching thresholds to exercise the speed dependent control function. The circuitry and associated electronic components necessary for the switching operations is very costly and not suited for large scale production.
According to British Pat. No. 1,379,606, a speed dependent control arrangement for power steering is disclosed wherein two solenoid valves of the open or closed types are utilized for converting speed responsive electrical signals into fluid flow control operations through four switching positions of the valves. The latter prior art arrangement is however relatively inflexible and not adaptable to different functional characteristics associated with various fluid power steering installations.
It is therefore an important object of the present invention to provide speed dependent power steering systems which differ in speed control influence, with a simplified, flexible and less costly control arrangement for the fluid flow controlling device associated therewith. A further object is to provide a simple and safe apparatus for controlling the fluid circuits of power steering systems with different operational characteristics in accordance with vehicle speed dependent signals from a speed sensor, wherein speed sensor failure will not prevent continued functioning of the power steering system. Yet another object is to provide such speed control apparatus which will function with relatively little heat development.